1. Field of the Invention
The invention relates generally to a wet gas probe. More particularly, this invention relates to a sensor probe capable of accurately measuring the temperature and liquid volumetric fraction of a hot gas laden with liquid droplets.
2. Description of Related Art
For many liquids, there is a temperature well above the boiling point called the Leidenfrost point or Leidenfrost transition. As an example, water has a Leidenfrost point of 300-350xc2x0 C. at atmospheric conditions. Consider a simple experiment where a droplet of water is placed on a surface kept at a temperature above the boiling point of water. If the temperature of the surface is below the Leidenfrost point, then the droplet starts to spread out and vaporizes rather quickly.
However, if the temperature of the surface is at or above the Leidenfrost point, the bottom layer of the droplet vaporizes almost immediately on contact, creating a cushion of vapor that repels the rest of the droplet from the surface. Furthermore, the evaporation of the bottom layer of the droplet from the surface produces a cooling effect, which detrimentally affects the heating of the surface. The remaining portion of the droplet does not make contact with the surface, and thus no heat can be transferred directly from the surface to the droplet. At such high temperatures, one might expect that the vapor layer would quickly transfer enough heat to the rest of the droplet to vaporize the droplet. Water vapor, however, is a very poor conductor of heat at these temperatures. Hence, the vapor layer actually acts as an insulator.
Currently it is quite difficult to accurately measure gas temperature in a liquid droplet and gas mixture. Most common gas measurement devices, such as thermocouples and resistance temperature detectors (RTD), work on the principal that the electrical resistance of most materials varies with temperature. Hence, knowledge of the functional relationship between temperature and change in resistance of a given material(s) and measurement of this change allows for inference of the temperature of the gas stream. Generally, these devices are comprised of different metals, which have high thermal conductivity. In a liquid-droplet gas mixture, the droplets will tend to impact and coat the surface of these devices. The liquid on the surface of the probe draws heat from the measurement device, in a process known as evaporative cooling. This process results in measurement of temperatures much below the true gas temperature. This phenomenon prevents these common temperature measurement devices from accurately measuring gas temperature in this environment. As described above, the invention eliminates this problem by preventing the droplet from impacting the surface of the probe by keeping the measurement surface above the Leidenfrost point.
The above-identified phenomena give rise to an unsolved problem in the prior art of measuring the temperature and other characteristics of gases laden with liquid droplets. The need to solve this problem arises in a wide variety of contexts, including, for example, measurement of gas temperature in propulsion and power generation systems, which often use water or other liquid introduced into the system to, for example, control emissions (e.g., pollution control) or augment power. In such applications, determination and control of gas temperature may be important to performance. Many other systems, devices, and situations arise, such as a gas turbine, combustion engine, tank, pipe, duct, manifold, chamber, or the like, as well as external flows, including, for example, droplets in an open air environment, in which the presence of liquid droplets in a gas can produce difficulties in measuring gas temperature and other properties of the gas.
As an example, one simple conventional temperature and liquid sensitive system for which this problem can arise is a fire detection system, an example of application of which is as follows. As is well known, fire detection systems are installed in residential and commercial buildings to protect property and occupants from fire. An important characteristic of the fire detection system is the capability to detect a fire in the early stages, when the fire is still small. Early detection and activation by fire suppression devices are important to allow more time for the evacuation of the occupants as well as to increase the chance of successfully suppressing the fire before extensive damage is caused to the buildings. Therefore, the early detection of a fire is very important.
Ceiling mounted devices that do not interfere with normal room arrangements are generally preferred for fire protection purposes. Automatic sprinklers are devices that distribute water onto a fire in sufficient quantity either to extinguish the fire in its entirety or to prevent the spread of the fire in case the fire is too far from the water discharged by the sprinklers. Typically, the water is fed to the sprinklers through a system of piping, suspended from the ceiling, with the sprinklers placed at regular intervals along the pipes. The orifice of the sprinkler head is normally closed by a disk or cap held in place by a temperature sensitive releasing element. The temperature sensitive releasing element of the sprinkler will be referred to hereinafter as a sprinkler link.
Automatic sprinklers have several temperature ratings that are based on standardized tests in which a sprinkler is immersed in a liquid and the temperature of the liquid raised very slowly until the sprinkler activates. The temperature rating of most automatic sprinklers is stamped on the sprinkler. The time delay between the onset of the fire and the activation of the sprinkler depends upon several parameters, such as the placement of the sprinkler with respect to the fire, the dimensions of the enclosed space, the energy generated by the combustion and the sensitivity of the sprinkler.
Buoyancy pushes the hot products generated by a fire toward the ceiling while mixing with room air to form a hot-gas plume. Impingement of the hot-gas plume on the ceiling results in a gas flow near the ceiling, even at a considerable distance from the core of the fire. This flow is responsible for directing hot gases to the thermally actuated fire detection devices.
The rate of heat released by the fire and the room dimensions are the main parameters of considerable importance in any discussion of fire-induced convection near the room ceiling. Also, the size and the composition of the sprinkler link influences the sensitivity of the sprinkler. Other conditions being equal, the sensitivity of a sprinkler is inversely proportional to the time required for the sprinkler link to melt. Therefore, sprinklers are rated according to their Response Time Index, hereinafter referred to as RTI, which characterizes the speed of the sprinklers response to a fire.
The RTI is the product of the thermal time constant of the sprinkler link and the square root of the flow velocity of the hot gas. This parameter is reasonably constant for any given sprinkler and is considered sufficient for predicting the sprinkler response for known gas temperatures and velocities near the sprinkler. However, recent full-scale tests on warehouse fires uncovered a behavior of sprinklers that does not correspond to the predictions of the RTI model.
The RTI model considers the sprinkler link as a cylinder in cross-flow. It is assumed that the heat transfer between the hot gases flowing under the ceiling and the sprinkler is convective and radiative, thus, among other factors, the RTI model neglects the presence of water droplets in the airflow.
The first sprinkler to activate in case of fire is referred to as a primary sprinkler and the surrounding sprinklers are identified as secondary sprinklers. Tests show that the primary sprinkler activates as predicted, but the secondary sprinklers respond after a much longer delay than suggested by the RTI model. In some cases, the sprinklers immediately surrounding the primary sprinkler do not activate at all, whereas the sprinklers farther away do activate.
Such observations may be explained in part by considering the presence of water droplets in the hot gas plume following the activation of the primary sprinkler. Some of the water droplets sprayed by the primary sprinkler do not reach the ground but are entrained and carried away by the ascending plume. Most of these water droplets evaporate inside the plume, while a small fraction of the remaining water droplets travel far enough to reach and impact the secondary sprinklers. The subsequent evaporation of the water droplets from the sprinkler link surface of the secondary sprinklers produces a cooling effect, which delays the heating of the sprinkler link. The delay in the heating of the sprinkler link results in a delay in the activation of the sprinkler which cannot be predicted by the RTI model.
There remains an unmet need to solve the problem of measurement of the characteristics, including gas temperature, for hot gases containing liquid droplets in a wide range of environments and applications.
The present invention solves these and other problems of the prior art by providing a sensor probe and method of use for determining the temperature and liquid volumetric fraction of a hot gas laden with liquid droplets in a wide range of applications. In one embodiment, a single heating element is used in a well-characterized, droplet-laden flow. The heating element is maintained above the Leidenfrost transition for the droplets which prevents cooling effects from the droplets from impacting the temperature measurement. In this embodiment, the heat loss from the probe is determined by the power needed to maintain a constant temperature in the element and the convective heat transfer coefficient is determined by calibration in the well-characterized flow or from fundamental properties of the well-characterized flow. Specifically, if the Reynolds number (Re) and the Prandtl number (Pr) of the flow are known, the convective heat transfer coefficient can be calculated from the Nusselt (Nu) number by a person skilled in the art. Similarly, if the temperature of the flow is known, the velocity of the flow can be determined through the velocity dependence of the convective heat transfer coefficient by calibration or from fundamental properties of the fluid. Such determination can include, but is not limited to, use of a processor, such as a personal computer, minicomputer, main frame computer, or microcomputer.
In another embodiment of the present invention, the heating elements are arranged so as to be in similar flow environments, such as by being arranged parallel, coplanar, or coaxial to one another. For example, in a single direction flow environment, the heating elements may be arranged coaxial to one another, perpendicular to the direction of flow. At least two heating elements are maintained at a temperature above the Leidenfrost transition for the liquid droplets (e.g., the Leidenfrost transition for water droplets is in a range between 300-350xc2x0 C. at atmospheric conditions; other liquids have determinable Leidenfrost transition temperature ranges), which prevents cooling effects of the droplets from impacting the temperature measurement. The gas temperature is derived by relying on thermodynamic and heat transfer principles, which are not usable in conjunction with conventional devices (e.g., devices designed for use at below Leidenfrost transition temperatures and for which the presence of liquid droplets impacts performance). In one variation, the temperature is determined using a relationship function for characteristics of two heating elements, maintained at two different temperatures, and the power needed to maintain a constant temperature in each element, and by eliminating dependence on other variables for the determination, such as the velocity of the gas, one important factor is heat loss from the heating elements.
More specifically, in an embodiment of the present invention, a temperature of the hot gas laden within liquid droplets is determined based on characteristics of heating elements in the hot gas and liquid droplet environment, such as by using a derived relationship function for the first and second temperatures of first and second heating elements positioned so as to be in a similar flow environment. Heat loss for the two elements is usable in this embodiment to determine gas properties, such as gas temperature. Heat loss can be measured in many ways. For example, if the heating elements are maintained at a constant temperature using a power source, heat loss varies as a function of the power supplied to each element, and the ratio of power supplied to the first and second heating elements by the controller is usable to determine relative heat loss, and from this information, gas temperature can be determined. As the derived relationship function eliminates variables for gas velocity, the determination of temperature is not affected by the velocity of the hot gas flow, and as the heating elements are both maintained above the Leidenfrost point for the liquid present in the hot gas, the presence of the liquid droplets in the hot gas flow likewise does not affect temperature determination.
In one embodiment of the present invention, heat loss is determined based on the power needed to maintain each heating element at a predetermined temperature, with the heat loss of the first and second heating elements and ratio of power supplied to the first and second heating elements being used to determine the temperature of the hot gas laden with liquid droplets, based on the relationship of TG=Axc2x7w+B, where TG is the temperature of the hot gas, A and B are numerical constants having values dependent upon the configuration of the sensor probes, and w is the ratio of an electrical resistance across the second heating element at the second temperature to an electrical resistance across a resistor that is in series with the second heating element.
According to yet another aspect of the invention, a liquid volumetric fraction of the hot gas laden with liquid droplets is determinable using information obtained from one or more of the heat elements and information obtained from a wetted sensor. In one embodiment, the liquid volumetric fraction of the hot gas is determined based on the relationship of xcex2=(0.012xc2x10.001)(TGxe2x88x92TW)/(Uxc2x7D)1/2, where xcex2 represents the liquid volumetric fraction, TG is the temperature of the hot gas laden with liquid droplets, TW is a temperature measured by a wetted sensor, U is the velocity of the hot gas laden with liquid droplets, and D is the outer diameter of the wetted sensor.
In one embodiment of the present invention, the controller controls the operation parameters of the first and second heating elements simultaneously (i.e., at the same time), using, for example, a circuit containing a Wheatstone bridge.
In an embodiment of the present invention, the sensor includes at least two heating elements connected to insulating rods attached to a support frame. A controller is connected to the support frame and controls the operation parameters of each of the heating elements, including the temperature of the heating elements. The first heating element is configured to be maintained at a first temperature and the second heating element is configured to be maintained at a second temperature that is less than the first temperature. The first and second temperatures are both above a Leidenfrost transition temperature at atmospheric conditions for the liquid present in the gas.
In an embodiment of the present invention, the first and second heating elements may be any of a wide variety of shapes, including cylindrical, spherical, or having a rectangular or trapezoidal cross-section, and may be, effectively, one, two, or three dimensional structures.
In one embodiment of the present invention for use in generally single directional gas flow applications, the first and second heating elements are arranged to be parallel relative to each other. An insulator is connected to each of the first and second heating elements that may be comprised of a rigid ceramic assembly. In another embodiment of the present invention for generally single directional gas flow applications, the first and second heating elements are arranged to be coaxial relative to each other. In this embodiment, a support frame is used that includes three bent rods connected at a first end of each rod and equidistant from each other at a second end of each rod. The second ends of the rods are provided on a line coaxial with the longitudinal axes of the first and second heating elements. A distance between the second ends of the rods is then equal to the length of the first and second heating elements.
A wide range of other configurations of the first and second heating elements are also usable in conjunction with the present invention, so long as each element is positioned in a similar flow environment (e.g., so that similar convective heat transfer error occurs for each element).
In an embodiment of the present invention, a connector may connect the first and second heating elements to the second ends of the rods. The connector may comprise a bore formed in the second end of each rod and a fastener having a threaded portion and a head portion. The bore can be sized and configured to receive the threaded portion of the fastener and the head portion of the fastener maintains the heating element against the corresponding rod. The fastener may also include a clamp having a bore identical to the bore in the second end of each rod and a terminal that maintains a predetermined distance between the clamp and rod. The bores can be sized and configured to receive the threaded portion of the fastener and the clamp maintains the heating element against the corresponding rod.
According to yet another aspect of the invention, the first and second heating elements may comprise a platinum wire. Furthermore, the support frame and connecting rods can be enclosed within temperature-shrinking tubing.
In another aspect of the invention, a method of measuring characteristics of a hot gas laden with liquid droplets uses a sensor probe having at least two heating elements. In one embodiment, each heating element is connected in series with an insulator and a support frame, and the insulator connected to each heating element is, in turn, connected to a support frame. A controller controls the temperature of each of the heating elements. A first heating element is configured to be maintained at a first temperature and a second heating element is configured to be maintained at a second temperature that is less than the first temperature, with both the first temperature and the second temperature being above the Leidenfrost temperature for the liquid present in the gas.
The method includes positioning the first and second heating elements in a flow of hot gas laden with liquid droplets, the elements being positioned in a similar flow environment, such as placement in parallel or coaxial, and orthogonal to the direction of the hot gas if the gas has a generally single flow direction. In one embodiment, power is then supplied from the controller to the first and second heating elements and the first and second temperatures are maintained above a Leidenfrost transition temperature for the liquid at atmospheric conditions. A temperature of the hot gas laden within liquid droplets is then determined, based on a relationship between a function of the first and second temperatures of the first and second heating elements and the power supplied to the first and second heating elements.
According to another aspect of the invention, the method also includes determining the temperature of the hot gas laden with liquid droplets based on the relationship of TG=Axc2x7w+B, where TG is the temperature of the hot gas, A and B are numerical constants having values dependent upon the configuration of the sensor probe, and w is the ratio of an electrical resistance across the second heating element at the second temperature and an electrical resistance across a resistor in series with the second heating element.
According to another aspect of the invention, the method also includes determining a liquid volumetric fraction of the hot gas laden based on the relationship of xcex2=(0.012xc2x10.001)(TGxe2x88x92TW)/(Uxc2x7D)1/2 where xcex2 represents a liquid volumetric fraction, TG is a temperature of the hot gas laden with liquid droplets, TW is a temperature measured by a wetted sensor, U is a velocity of the hot gas laden with liquid droplets, and D is an outer diameter of the wetted sensor.